pH sensors includes one that is miniaturized by using, not a conventional glass electrode but an ISFET (Ion Sensitive Field Effect Transistor) chip that is a sort of sensor chip for electrochemical measurement. For example, a pH sensor described in Patent literature 1 is configured such that a thorough-hole is formed through a wider width side surface of a body having a substantially flat rectangular parallelepiped shape, and from the through-hole, a sensing part of an ISFET chip is exposed. This pH sensor is one that is placed sideways on a desk or the like, and measures pH by dropping a minute amount of sample liquid into the through-hole with a dropper or the like, or is used with the through-hole and the ISFET chip being immersed in a liquid measuring object.
Also, pH sensors 200A that are configured to be placed on a desk or the like as the pH sensor 200A described above and bring a measuring object into contact with a sensing part of an ISFET chip 3A include a pH sensor 200A described in Patent literature 2. This pH sensor 200A is configured to make it easy to bring a solid such as skin into contact with a sensing part of an ISFET chip 3A, and in order to decrease a thickness thereof, configured such that, as illustrated in a partial cross-sectional view of FIG. 8, a front side of a thin substrate 2A is set as a surface that comes into contact with a measuring object M, and the ISFET chip 3A is attached on a back surface of the thin substrate 2A so as to expose the sensing part from a through-hole. Also, on the back surface of the thin substrate 2A, wiring for taking out an output signal from the ISFET chip 3A is surface-mounted. Such a configuration is also presented in the pH sensor 200A described in Patent literature 1. That is, it turns out that the configuration in which the ISFET chip 3A is attached on the back surface of the substrate 2A is used in the case where there is a technical subject of desirably decreasing the thickness of the pH sensor 200A or making it easy to come into contact with the measuring object M, in particular, in the case of configuring the pH sensor 200a of a flat plate type.
Meanwhile, the pH sensor 200A as described above is intended to be placed and move the measuring object M to the pH sensor 200A side to make the measurement; however, even in a probe type pH sensor 200A that is mainly intended to be moved, and immersed in or brought into contact with the measuring object M by a measurer, the ISFET chip 3A may be used.
Further, Non-patent literature 1 describes a pH sensor 200A that is formed in a similar manner as that for the above-described one, and intended to prevent the measurement from being blocked by air bubbles that cover a sensitive part of an ISFET chip 3A when the pH sensor 200A is immersed in a liquid measuring object. As illustrated in FIG. 9, this pH sensor 200A is configured such that a fore end surface S formed in a fore end part of a rod-like body 1A is formed so as to form an angle of 45 degrees with a virtual plane V that is vertical to an axial direction of the body 1A, and in the central part of the fore end surface S, the ISFET chip 3A is arranged. More specifically, a lead L connected to the ISFET chip 3A is sealed in a space near a central axis of the body 1A by a first sealing material 61A, and in order to completely insulate the lead L that is drawn to an upper part of the ISFET chip 3A placed on the fore end surface S, a second sealing material 62A is placed around the ISFET chip 3A to thereby form the fore end surface S. In addition, internal liquid 16 is contained in a space around it, and a liquid junction part 4A that electrically connects the internal liquid 16A and a measuring object M to each other is formed in a side surface part of the body 1A. As described, the probe type pH sensor 200A is not required to be thinly configured like the above-described flat plate type pH sensor 200A, so that an output from the ISFET chip 3A is taken out by the lead L, and the lead L is insulated by placing the sealing material such as epoxy resin.
However, even in the above-described pH sensor 200A, it is difficult to prevent air bubbles from accumulating in the sensing part of the ISFET chip 3A on the fore end surface S at the time of liquid immersion. The present inventors have extensively examined the reason why such a phenomenon occurs, and as a result, first found that, in order to insulate the lead L connected to the ISFET chip 3A from the measuring object, the sealing material is placed, so that as illustrated in FIG. 9, a hole from the fore end surface S to a part where the sensing part of the ISFET chip 3A is exposed becomes deeper, and thereby sometimes, air bubbles supposed to move away along an incline of the fore end surface S accumulate in the hole to block the measurement from being made.